The basic chromatography is the separation of components of a sample owing to their differences in solubility or in adsorption in a stationary bed of a material (either liquid or solid). When the sample (moving phase) is a gas, the technique is referred to as gas-solid or gas-liquid chromatography, depending on whether the stationary phase is a solid or a liquid. In gas chromatography, a sample is introduced into a carrier gas as a vapor which flows through a chromatographic system. Upon separation by the stationary phase, the analytes travel through the gas chromatograph at different speeds and enter a detecting device, which device is connected to the gas chromatograph, at different times. As a result, individual analytes that are present in the sample may be identified by the detecting device.
The analytes transported carried using a carrier gas. The carrier gas is an inert gas for the analyte. Argon and helium are two examples of carrier gases. Other gases and mixtures of gases can be used as well, depending on the implementations and/or the requirements.
A same gas chromatograph can be used with different kinds of detecting devices, depending on the needs. The various kinds of detecting devices can themselves have different sensitivity levels. For instance, some detecting devices can be designed to detect very low concentrations of an analyte, such as in the range of parts per million (ppm). Others can be designed to detect concentrations in the range of a few percent or more.
Some detecting devices can measure the concentrations of analytes based on ionization. The carrier gas with the analytes is directed from the outlet of the gas chromatograph to an ionization chamber located in-between a pair of electrodes provided inside the detecting devices. The detecting device is designed to transform the carrier gas and each analyte into plasma using the electrodes. The plasma results in light radiations, including visible light. The light radiations can be sensed and recorded using a corresponding light sensor. The spectral content of the data obtained from the light sensor can reveal the presence of some analytes and their concentration.
In general, the size of a detecting device is a factor that can impact the operation of gas chromatographs. Larger detecting devices require more space next to the gas chromatograph and can also require a relatively high flow rate. A higher flow rate means that more carrier gas must be used and this increases operation costs. Still, minimizing the flow rate is further desirable given all the usual inherent difficulties in alleviating contamination of the carrier gas and the whole carrier gas circuit. Minimizing the size of detecting devices is thus generally desirable. It is also desirable to minimize the size of detecting devices since the available space around gas chromatographs can be limited.
Permeation devices for adding OH doping agent to the carrier gas are used in the field of gas chromatography to increase the accuracy of the measurement of analytes. Most permeation devices use water provided inside a semi-permeable membrane as a source of OH doping agent. For instance, adding, accurately adjusting and maintaining the level of water vapor in the carrier gas improves and/or stabilizes the carbon impurities that may be present in the carrier gas circuit, thereby allowing them to be measured. OH doping agents can partially reduce or even totally eliminate the carbon deposits that tend to adhere on the walls of the discharge zone. Over time, these carbon deposits can block the light radiations from the sensors and shorten the lifespan of the detecting device.
Various configurations and arrangements exist to provide OH doping agents in the carrier gas. Existing permeating devices require additional external hardware components and corresponding control systems. For instance, ovens can be used to provide heat for controlling the amounts of water vapor going through the semi-permeable membrane of the permeation device. However, this adds complexity and leaves less available space around the gas chromatograph. Relatively small permeation devices exist but these devices are tailored to specific concentrations and/or flow rates. They also have a limited lifespan since only a relatively small quantity of water is present therein and they cannot be refilled. Switching from one permeation device to another is generally a difficult task in gas chromatography.
Accordingly, there is still room for many improvements in this area of technology.